1. Field of the Invention
This invention generally relates to a friction material. More specifically, the present invention relates to a cermet friction material or a metallic friction material made of a copper or iron alloy.
2. Background Information
A clutch device of a vehicle is attached to a flywheel on the engine side and functions to switch between a state in which torque is transmitted from the flywheel to a main shaft of a transmission and a state in which the torque is disengaged. The clutch device basically includes a clutch disc assembly and a clutch cover assembly. The clutch disc assembly basically includes a hub, a plate member, and a frictional holding member. The hub is coupled to the main drive shaft that extends from the transmission. The plate member is coupled to the hub in such a manner that it can transmit torque to the hub. The frictional holding member (clutch disc) is fixed to the outer circumference of the plate member. The frictional holding member typically has a ring-shaped plate and a pair of friction facings (friction material) that is fixed to the side surfaces of the plate. Existing friction materials used for the friction facing include copper-based or iron-based metallic friction materials and cermet friction materials.
Metallic friction materials and cermet friction materials utilize the heat resistance and high surface pressure characteristics of metal and are used on the clutch discs of clutch devices in heavy vehicles and race cars. Friction facings made of these friction materials are manufactured by heat-sintering a mixture containing prescribed materials and sintering the mixture to a core plate.
For example, in a copper-based friction material, the main ingredient is copper and the copper is mixed with such additives as graphite and ceramic. Graphite is a lubricating ingredient added to provide smooth operation. Silica and other ceramics are added to improve frictional resistance retention and wear resistance.
A cermet is a monolithic material composed of a hard ceramic component and a binder component. The hard ceramic component can typically comprise a nonmetallic compound or a metalloid. The hard ceramic component may or may not be interconnected in two or three dimensions. The binder component can typically have a metal or alloy that is generally interconnected in three dimensions. The binder component cements the hard component together to form the monolithic material. Each of the monolithic cermet properties is derived from the interplay of the characteristics of the hard component and the characteristics of the binder component. For example, if the hard component or the binder component exhibits ferromagnetic characteristics so might the monolithic cermet.
The metallic friction materials and cermet friction materials described above have excellent heat resistance and wear resistance, but the xcexc-v characteristic of each has a negative slope. In other words, when the friction material slides along the mating material, the coefficient of friction increases as the relative velocity between the materials decreases. Consequently, when these materials are used on the clutch discs of clutch devices or the brake material of brake devices, sticking and slipping can easily occur. This phenomenon is known as so-called juddering.
In order to alleviate juddering, substances such as lead, which are considered to be harmful, must be contained in the friction material to secure better lubrication. However, out of concern for the environment, there is a demand for a friction material that does not use lead and other harmful substances.
In view of the above, there exists a need for a friction material which overcomes the above mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
An object of the present invention is to improve the xcexc-v characteristic of a friction material without using lead and other harmful substances.
The friction material of the present invention is a cermet friction material or a metallic friction material made of a copper alloy or an iron alloy, wherein the material contains about 1 to about 20 wt % of graphite and about 1 to about 15 wt % of ceramic. The graphite has a particle size of about 50 xcexcm to about 200 xcexcm and is arranged in a layer-like form. The ceramic has a particle size of about 50 xcexcm to about 200 xcexcm.
According to an aspect of the present invention, about 1 to 20 wt % of graphite is included as a lubricating ingredient and about 1 to about 15 wt % of ceramic having a particle size from about 50 xcexcm to about 200 xcexcm is included to secure frictional resistance retention and wear resistance. In order to improve the lubrication in comparison to conventional friction materials, the graphite is arranged in a layer-like form. With the graphite arranged in a layer-like form, the layers peel off one by one when the frictional sliding causes the graphite to exfoliate. Therefore, lubrication is maintained even if some of the graphite exfoliates. Conversely, in the case of conventional friction materials containing block-shaped graphite, the sliding surface becomes rough and lubrication is greatly reduced when the graphite exfoliates.
The particle size of the graphite is set to about 50 xcexcm to about 200 xcexcm because it is extremely difficult to arrange the graphite in a layer-like form if the particle size is smaller than about 50 xcexcm. Further, there is too much slipping if the particle size is larger than about 200 xcexcm. Thus, by arranging the graphite in a layer-like form, the friction material described in this claim is able to improve the lubrication in comparison with conventional friction materials and improve the xcexc-v characteristic of the friction material without including lead or other harmful substances. Moreover, the xcexc-v characteristic of the frictional material can be given a positive slope by adjusting the content and particle size of the graphite. When the xcexc-v characteristic of the friction material has a positive slope, it is more difficult for juddering to occur.
According to another aspect of the present invention, the graphite of the friction material of the present invention is preferably natural graphite. Roughly speaking, graphite can be divided into natural graphite and artificial graphite. In this aspect of the present invention, natural graphite, which is easy to arrange in layer-like form (readily assumes a layer-like form), is contained in the friction material as a lubricating substance.
According to another aspect of the present invention, the ceramic of the frictional material of the present invention is a single substance made of silica, mullite, or zircon sand or a composite containing at least two of silica, mullite, and zircon sand.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.